Re: Patch to add RAID autostart to IBM partitions

[Neil Brown]

On Saturday October 16, zaitcev@xxxxxxxxxx wrote:
> On Sat, 16 Oct 2004 12:09:39 +0100
> Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> 
> > We had this for a few partition types (and full block devices) already
> > and it's been vetoed.  Use mdadm from initrd/initramfs instead.
> 
> It is doable in theory, but in practice mdadm is too retarded (I'm taliking
You sweet-talker you....

> about mdadm 1.6.0 here). Its man page says:
> 
>     mdadm can perform (almost) all of its functions  without  having  a
>     configuration  file  and  does  not use one by default.
> 
> I guess "almost" is the key. Firstly, it collapses with any attempt to run
> "mdadm -A -s" or any variation of such, because it's full of places like
> this.
> 
> 	mddev_ident_t array_ident = conf_get_ident(configfile, dv->devname);
> 	mdfd = open_mddev(dv->devname, array_ident->autof);

Hmm, thanks for the bug report.  I'll fix that rather silly bit of
code..... there.  The next release won't have that bug.
> 
> Obviously, array_ident is NULL if a config file is not present. It is
> possible to fool the stupid thing by giving it a dummy config file,

There's that silver tongue again.  It makes one feel all warm
inside....

> like this one:
> 
> DEVICE partitions
> ARRAY /dev/md0
> 
> It still won't scan, aborting with this:
> mdadm: /dev/dasda has no superblock - assembly aborted

Yes, you haven't told it what defines "/dev/md0", so it assumes the
first device listed will define it, but /dev/dasda doesn't so....

You have to tell mdadm how to recognise /dev/md0.  Possibly by UUID.
Possibly by "minor number".
e.g.

 DEVICE partitions
 ARRAY /dev/md0 super-minor=0

which means "if you find any devices listed in /proc/partitions which
contains an MD superblock which has a "md_minor" field of "0", then
use those to assemble /dev/md0.

This will often be what you want, but might not always.  e.g. if you
have plugged in a drive that was part of /dev/md0 in another machine,
then mdadm will have no way of knowing which drive is the "right" one.

NeilBrown
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